Action replay is exactly like gameshark just better because it's more popular, easy to use, more codes, etc.You can buy it at EB games/gamestop that's where I got mine.

After you have gotten it you can enter AR codes from this site to give your self a ton of rare candies and level up your pokemon so you can beat the elite 4. I would suggest just training them though because they miss out on EV training if you use rare candies.